The present invention relates to a light beam scanning unit for scanning an object with a light beam emitted from a light source in a predetermined direction through optical components including a deflecting member and a lens. This invention also relates to an image forming apparatus configured to perform electrographic image formation by irradiating an image carrier with a light beam emanated from a light beam scanning unit and modulated in accordance with image data.
In an image forming apparatus for electrophotographic image formation such as a copier or a printer, the surface of an image carrier electrostatically charged beforehand by static charger means is exposed to and scanned with a light image modulated in accordance with image data to form an electrostatic latent image thereon. The electrostatic latent image on the surface of the image carrier is turned into a visible toner image by development means and then transferred to a recording sheet. Finally, the toner image thus transferred is fixed to the recording sheet by fixing means.
Such an image forming apparatus has a light beam scanning unit as means of irradiating the surface of the image carrier with a light image. In general, frequent use is made of light beam scanning units of the laser beam type having a semiconductor laser for emitting laser light modulated in accordance with image data because such laser beam type light beam scanning units are suitable for increased modulation speed and scanning speed.
Such a light beam scanning unit has a housing accommodating therein a light source such as a semiconductor laser, as well as optical components including a collimator lens, polygonal mirror, fθ lens and reflecting mirror. The light beam scanning unit emanates a light beam from an emanation window having a glass plate to scan an object such as an image carrier. A problem with such a light beam scanning unit is that optical characteristics of the optical components are degraded because of condensation on the optical components including the emanation window which is likely when the temperature difference between the interior and the exterior of the housing becomes large. Specifically, as the ambient temperature rises rapidly after the light beam scanning unit has been allowed to stand at a low ambient temperature for a long time, outside air flowing into the housing of the light beam scanning unit contacts the surface of each optical component still remaining in a low-temperature state and is thereby cooled rapidly, with the result that the moisture contained in air condenses on the surface of the optical component.
In attempt to overcome this problem, one conventional light beam scanning unit is provided with a condensation preventing heater operative to prevent the temperature within the housing from becoming lower than the ambient temperature, as disclosed in Japanese Patent Application Laid-Open No. S63-137206.
Recently, however, the material of such a housing as a base component of the light beam scanning unit has been changed from diecast metal having a low suitability for volume production to a molded resin having a superior suitability for volume production in order to reduce the cost. Accordingly, the heat resistance of the housing becomes relatively low. For this reason it is impossible to mount the aforementioned condensation preventing heater directly on the housing. The housing is imparted with the function of fixing the aforementioned optical components to respective predetermined positions with high precision so as to obtain desired optical characteristics as well as the function of protecting the optical components so as to keep the obtained optical characteristics over a long term. If the heater adapted to generate heat to a high temperature is mounted for preventing condensation directly on the housing comprising a molded resin product having a low heat resistance, the housing will be deformed to cause deviation of the optical axis of a laser beam, incorrect focusing, degradation in fθ characteristic, and the like, thus resulting in a problem that desired optical characteristics cannot be ensured.
Further, a heater dedicated to heat the optical components has to be disposed at an optimal position within the housing, which leads to increase in the size of the housing and in the cost.
A feature of the present invention is to provide a light beam scanning unit which is capable of preventing condensation on the optical components without provision of a dedicated heater on the housing, preventing degradation in image quality due to deformation of the housing, and avoiding the increase in the size of the housing and in the cost. Another feature of the present invention is to provide an image forming apparatus provided with such a light beam scanning unit.